Starter devices for asynchronous three-phase motors comprising semiconductor devices for controlling the voltage applied to the motor in one, two or three of the phases by adjusting firing angles of semiconductor devices of the type turning-off at zero-crossing of the current therethrough, two connected in anti-parallel to each other in each phase, have been used for some time. By controlling said voltage in the respective phase a reduction of the torque and current provided to the motor is achieved during start and stop operation. The firing angle of the semiconductor devices is used for controlling the amount of energy supplied to the motor.
These types of starter devices, often called softstarters, are normally provided with three such pairs of semiconductor devices, such as thyristors. However, these semiconductor devices have become the cost-determining factor of such starter devices, so that such starter devices having only one pair of such semiconductor devices for two of the phases are also used. This means that the remaining third phase is in the form of a conductor, which cannot be switched. The present invention is occupied with this type of starter devices.
By adjusting the firing angles of the semiconductor devices in two of the three phases the voltage across the motor may be controlled from zero to 100% of the maximum voltage thereacross. This is done by changing the firing angle of the semiconductor devices, such as to reducing it for increasing said voltage. However, when the control takes place in only two of the phases, unwanted effects in the form of for example a DC component in the motor current appear. During start up operation a smooth and gradually increasing torque is desired, but this DC component appearing during start up sometimes creates a braking or oscillating torque. This can be both disturbing and damaging to the controller system, the electric motor, and to the motor driven load.
The present invention is directed to a method and an apparatus for reducing the influence of such a DC-component in a load current of an asynchronous three-phase motor, in which the voltages of two of the three phases are controlled according to the aforesaid. The invention is not restricted to such methods and apparatuses for any particular such electric motor with respect to the motor voltage, but 200-600 V may be mentioned as a typical motor voltage, or the power delivered by such a motor, which is often in the range of 1-30 kW. Nor is the present invention restricted to such motors for any particular type of load, but the motor may be used to drive all types of equipment, such as pumps, turning machines and the like.
U.S. Pat. No. 6,930,459 discloses a method and an apparatus for reducing the influence of a DC component in the load current of an asynchronous three-phase motor, in which the voltages of two of the three phases are controlled by adjusting the firing angles of semiconductor devices as described above. However, the methods described therein are rather complicated with respect to measurements and calculations. Turn-on as well as turn-off times of the different semiconductor devices have to be detected and considered during the calculations of how the firing angle of the semiconductor devices shall be changed for reducing the influence of a said DC component.